You shouldn't need SMTP for the sendgrid mailer. It uses the sendgrid API to send mail messages, and optionally, allows for an inbound webhook to receive messages. Have you looked at the activity logs in the sendgrid console to verify that your requests are being handled? If needed, you can add a console log to the Emailer.send method in the nodebb-plugin-emailer-sendgrid index.js file if you want to debug the message handling.

Would it be possible to add a line calling that out explicitly in https://docs.nodebb.org/configuring/config to make it obvious to stupid people like me?